feat: voice channel user limit (P5-10) + call join/leave sounds (P5-16)

P5-10 Voice Channel User Limit:
- New StateEvent.LotusVoiceLimit (io.lotus.voice_limit) with { max_users }
- RoomVoiceLimit admin control in Room Settings > General > Voice
  (power-level gated via permissions.stateEvent)
- CallPrescreen reads the limit reactively and disables Join with a
  'Channel Full (N/N)' message at capacity; existing members can rejoin

P5-16 Custom Join/Leave Sound Effects:
- useCallJoinLeaveSounds hook wired into CallUtils; detects participant
  join/leave via MatrixRTCSession membership changes (sender|deviceId),
  filters out self, only fires while joined
- Sounds synthesized in-browser with Web Audio (callSounds.ts) - no
  assets bundled; styles Off/Chime/Soft/Retro
- 'Join & Leave Sounds' selector in Settings > Calls (previews on change)

Docs: LOTUS_FEATURES.md, README.md, LOTUS_TODO.md (P5-10/P5-16 marked done)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/components/CallEmbedProvider.tsx

@@ -42,6 +42,7 @@ import { ScreenSize, useScreenSizeContext } from '../hooks/useScreenSize';
 import { useMatrixClient } from '../hooks/useMatrixClient';
 import CallSound from '../../../public/sound/call.ogg';
 import { useCallMembersChange, useCallSession } from '../hooks/useCall';
+import { useCallJoinLeaveSounds } from '../hooks/useCallJoinLeaveSounds';
 import { useRemoteAllMuted } from '../hooks/useCallSpeakers';
 import { useRoomAvatar, useRoomName } from '../hooks/useRoomMeta';
 import { mDirectAtom } from '../state/mDirectList';
@@ -406,6 +407,7 @@ function CallUtils({ embed }: { embed: CallEmbed }) {
   const setCallEmbed = useSetAtom(callEmbedAtom);
 
   useCallMemberSoundSync(embed);
+  useCallJoinLeaveSounds(embed);
   useCallThemeSync(embed);
   useCallHangupEvent(
     embed,